Grain refinement is a technique for improving the characteristics of metal alloys. Fine grained solidification structures possess improved properties for subsequent working and generally improved strength in the final product. A method of refining grain size in some metals is addition of grain refining compound (e.g., TiB in the case of aluminum alloys). In other cases, notably iron-carbon alloys, no effective grain refining agent exists.
A grain refining method that can be used for improving iron-carbon alloys is to heat the alloy to a temperature above its liquidus temperature, followed by stirring and controlled cooling of the melt to below its liquidus temperature, by, for example, pouring the alloy at a temperature near its liquidus temperature into a cooling vessel, so that initial solidification takes place while convection from the pouring remains significant. Significant process limitations, however, include the difficulty in controlling heat removal from the vessel walls and in sustaining sufficient convection to effect the desired formation of solid particles and their uniform distribution in the melt. Convection can be introduced by mechanical or electromagnetic methods, as described in, for example, U.S. Pat. No. 4,567,937 to Ujiie et al., issued Feb. 4, 1986, but these methods add to the cost and are not always effective.
Semi-solid metal alloy compositions with a relatively high solids content (e.g., greater than about 10 wt %) are desirable because they are thixotropic and therefore can be cast into molds and handled as solids before the molded shapes are fully solidified, increasing the number of castings produced per mold per given amount of time, as described in U.S. Pat. No. 3,954,455 to Flemings et al. issued May 4, 1976. Nevertheless, a considerable amount of time is typically required to cool the slurry to the desired solids content.
Therefore, a method is needed to produce solid particles in metal alloys that overcomes or minimizes the above-referenced problems.